Catalysts are employed in a variety of chemical processes such as synthesis and reforming of hydrocarbons, ammonia synthesis, etc. Many such catalysts consist of or contain metallic, i.e., nickel, iron, copper, chromium, etc., compounds which may be either temporarily deactivated or permanently poisoned by certain contaminants or components in a process stream. Prominent among the permanent poisons are hydrogen sulfide and various organic sulfur compounds. With such catalysts it is essential that the sulfur be removed from the process stream to prevent contact with such catalysts.
Various methods for removing sulfur have been proposed and several are in current use including washing with sulfuric acid, absorption on activated carbon and absorption or chemisorption by various materials. With high-sulfur feedstocks, a combination of desulfurizing methods is sometimes employed.
A material found to be particularly effective in reducing sulfur in process streams to an acceptable level, is zinc oxide. In U.S. Pat. No. 1,868,096 the process disclosed includes passing a stream of water gas over one or more masses composed of zinc oxide or zinc oxide and chromium oxide before contact with a methanol catalyst which is sensitive to sulfur poisoning. When used as a desulfurant, zinc oxide is converted to zinc sulfide. The active life of a charge of zinc oxide in a desulfurizing reactor is obviously dependent upon the sulfur content of the feedstock being processed. Nevertheless, maximum utilization of the zinc oxide itself is also of importance. In that regard, the availability of the zinc oxide for the desulfurizing reaction and the quantity of zinc oxide that can be contained within an available reactor volume have important effects upon the active life of the zinc oxide charge.
The amount of zinc oxide that can be contained in a given volume depends upon its purity and its bulk density. A product of low purity has an inherent disadvantage in that inactive impurities will occupy a portion of the reactor volume in more or less direct proportion to their percentage in the zinc oxide product. Hence, a zinc oxide of high purity is most desirable. The weight that can be charged into a reactor is directly related to its bulk density, that is, the weight of a given volume of product. Light, fluffy zinc oxides are at a stoichiometric disadvantage in that relatively small weights of such products will occupy the available reactor volume.
The availability of the zinc oxide for reaction with hydrogen sulfide and other sulfur-containing compounds is a function of its specific surface area, that is, the surface area of one gram of solid; at times, this is referred to simply as surface area. It is known that solids in the form of fine particles exhibit larger specific surface areas and greater reactivity than those of greater particle size and that solids can be produced in an "active" or highly dispersed state by thermal decomposition of a parent substance from which a volatile component is released. In U.S. Pat. No. 1,878,390 relating to the preparation of catalysts suitable for methanol production, it is disclosed that catalysts comprising zinc oxide or a mixture of zinc oxide and chromium oxide are particularly effective when prepared by heating the basic carbonates. The preparation of a highly reactive zinc oxide of exceedingly small particle size by the heating of ammoniated zinc carbonate is disclosed in U.S. Pat. No. 2,898,191 and Russian Pat. No. 308,976. The advantages of controlling the surface area of zinc oxide for use in removing sulfur compounds from industrial gas streams wherein steam in admixed is disclosed in U.S. Pat. No. 3,441,370; zinc oxides having surface areas above 30 square meters per gram are said to be of particular advantage. The preparation of these high surface area zinc oxides is by a wet process including precipitation of zinc carbonate followed by calcination for conversion to zinc oxide.
Although high specific surface area is a prime requisite of the zinc oxide intended for use in desulfurizing industrial gas streams, it is equally important that the zinc oxide be in a form suitable for charging into reactor towers. The physical form should be such that adequate resistance to handling, rather rough at times as in shipping, is provided. High bulk density is also desired in order that an adequate weight can be packed into a reasonable volume. Since the latter is variance with the requirement of high surface area for availability and reactivity, in that exceedingly finely divided solids are usually of low bulk density, agglomeration of high surface area products into a form suitable for charging into desulfurizing reactors is required. Various methods of agglomeration have been proposed including forming a paste and extruding it into cylinders which can be cut to desired lengths, forming tablets in a press, forming spherical pellets either in rotating drums or on rotating discs, etc. Normally an appreciable proportion of a binding material is required for adequate strength, particularly with high surface area zinc oxide, whereby the zinc oxide content is proportionately reduced. Optimum life of a zinc oxide product intended for use in desulfurizing industrial gas streams requires consideration of all of these factors.